ReactPHP is a low-level library for event-driven programming in PHP. At its core is an event loop, on top of which it provides low-level utilities, such as: Streams abstraction, async dns resolver, network client/server, http client/server, interaction with processes. Third-party libraries can use these components to create async network clients/servers and more.
React PHP libraries were introduced into cAPI release v1.3.2 to open the door for event-based application development.
https://learn.getcapi.org/advanced/event-driven-application-development